Integrated circuit semiconductor devices are typically made using photoetching techniques in which a photoresist is exposed to a light source through a working photomask. The working photomask conventionally is a glass plate having a pattern of opaque areas photographically developed thereon. It is formed by irradiating a photodarkenable emulsion on the glass plate through a master mask that has been made by photoetching techniques. The master mask usually is made by a more expensive process that involves photoetching a chromium layer on a glass substrate. The resist used to etch the chromium layer does not have to be opaque. Accordingly, it does not need to contain grains of a material which will darken upon irradiation and development. Accordingly, where extremely high dimensional accuracy is desired, one may choose to use a chromium photomask as the actual working mask. However, working masks, especially contact working masks, tend to deteriorate with use. Use of the more expensive chromium masks as working masks is ordinarily economically impractical, especially if they are to be used as contact working masks.
A recent report discloses using a normally transparent photoetching photoresist, such as Shipley's AZ-1350 and Tokyo Oka Kogyo's OMR-83 or OSR, as an opaque emulsion for a working mask. The report indicated that the photoresist was hardened by ion implantation and optically darkened enough to block out ultraviolet light above wave lengths of 300 nanometers. The resultant film therefore did not require grains of photodarkening material to produce opacity. Film pattern resolution is therefore not limited by the size of such grains, and dimensional accuracy is improved. In addition, the cost of making the photomask is reduced, as compared to photoetching processes.
The aforementioned photoresist films have base polymers that are respectively cresol novolac resin, cyclized polyisoprene, and poly (-vinyloxethyl cinnamate). Such photoresists are sensitive to visible light and to near ultraviolet light. Upon exposure to such light, through a mask, a pattern is activated in the transparent resist. The exposed portions of the film are selectively washed away when the film is rinsed in an appropriate solvent. Patterned photoresist coatings of up to about 4000 angstroms were darkened and hardened by ion implantation at energies of about 20-180 keV at a rate of about 0.16-1.25 microamperes per square centimeter in dosages of about 10.sup.14 -10.sup.16 ions per square centimeter.
In my aforementioned concurrently filed U.S. patent application Ser. No. 144,579, disclose how an electron sensitive resist can also be satisfactorily darkened to ultraviolet radiation and made scratch resistant by ion implantation. Electron sensitive resists are of interest because they can be patterned by a 0.1 micrometer wide electron beam, which inherently produces very high resolutions. However, a thicker coating and a different ion implantation procedure is required to darken and harden the electron resist. In Ser. No. 144,579 I propose a two-stage ion implantation process that initially darkens and hardens the electron resist, and then makes it extremely adherent to its glass substrate. Since the resist pattern is delineated by electron irradiation, an extremely high degree of dimensional accuracy is inherently available. My two-step ion implantation process does not significantly degrade it. Consequently, extremely narrow line widths, i.e. of the order of 0.5 micrometer and less, can be readily obtained. Master photomasks of high dimensional accuracy are obtained, in a simple process, suitable for use as master photomasks. Such master photomasks can also be used as working photomasks, if desired. However, it is quite costly to generate the pattern in the resist by electron beam irradiation. Hence, use of an electron beam generated photomask as a working mask is not practical at this time.
On the other hand, it has recently been reported that the electron resists such as polymethyl methacrylate (PMMA) and polymethyl-isopropenyl-ketone (PMIK) can be satisfactorily activated by deep ultraviolet light sources, 220 nanometers and 253.3 nanometers in wave length, respectively. My two-stage ion implantation of Ser. No. 144,579 can be used on such resists, regardless as to whether they have been patterned by electron beam or ultraviolet radiation. Pattern definition by ultraviolet radiation cannot provide as fine a resolution as obtained by electron beam irradiation. However, it is a considerably quicker and less expensive process, and the resolutions attainable are more than adequate for most present integrated circuit designs. The deep ultraviolet radiation to which PMMA and PMIK are sensitive inherently provides almost twice the resolving power resolution as the near ultraviolet radiation now typically used for photoresists such as the aforementioned AZ-1350, OMR-83 or OSR. Hence, I contemplated using my two-stage ion implantation to also make working photomasks of PMMA, PMIK or the like. However, the patterns of the working masks would be generated by ultraviolet radiation, not by an electron beam.
In my aforementioned Ser. No. 144,579, I describe but do not claim using master masks made by my process to produce working masks made by my process. I mentioned that if the resist is implanted as described herein, it will satisfactorily block ultraviolet light down to about 200 nanometers. Using such a short wave length of light to irradiate a resist inherently allows higher resolutions to be obtained. Hence, an extremely fine resolution master photomask made in accordance with this invention can be used with deep ultraviolet light to make a fine resolution working photomask in accordance with this invention. Morever, if this same resist is used on the silicon slice itself, and also irradiated with deep ultraviolet light through the aforementioned working photomask, very high resolutions are readily obtained on the slice itself. This is particularly beneficial because these results are attainable in a contact printing operation. This obviates the need for considerably more expensive projection alignment systems.